Method of forming fiber optical devices



Allg- 1961 J. w. HICKS, JR., EIAL 2,995,970 I METHOD OF FORMING FIBER OPTICAL DEVICES Original Filed April 11, 1958 INVENTORS JOHN W. HICKS, JB. W/LFEED P BAZ/NU JR QTTOQN Y United States Patent- 'Thisinvention relates to fiber optical image trwifer .devices and ,has particular reference to .angirnproved method of making op ahmasnityin r emas if ne devices formed of a multiplicity of light-conducting fibers or filaments in bundledrelation with each other for use in transferring optical images from one location to another and method of making the same.

"The instant application is a division of applicants copending application Serial No. 7,27 ;904ffiled April 11, "19,58.

' Fiber optical devices of the above general character embodying various arrangements of light-conducting fibers or elements which are tapered throughout their- "lengths have been used heretofore as qpticalimage magnifiers--with limited degrees of success. While ;such de- -vices'haveprovidedrelatively economical and compact ineans for magnifying optical images "the magnified images which'have resulted 'fronrtheuse ;of conventional fiber optical magnifiers have been lacking in definition :and contrast. It has beenfound that one of the causes 20f relatively poor contrast is "the dilution of -the f image forming light travelingthroug'h such devices by;

extraneouslight which is pickedup thereby and trapped the fibers. 'Moreover, inusiixg-cla d-or coatedlightconducting fibers to 'form the bundle, some of the light whichenters the coating at the light 'entrance'en'd of the device is transferred to-the cores of the fibers -andalso :trapped therein thus causing a further dilution of the image forming light passing thnough the cores of the fibers with the result that-the finally transferred optical image becomes deteriorated or lacking in contrast.

'The present inventionprovides means andmethod for :overcoming the above difiiculties 'by providing superior fiber optical image magnifiers'or demagnifiers which are capable of accurately transferring optical images with substantially no deterioration or 'loss 'of-contrast in the ttrsultant image which is transferred thereby.

It is accordingly an object ofthe present 'inventionto provide a simple and efiicient method 'of making devices of the above character for :accurately 'reproducing and magnifying or demagnifyingoptical images while transferring said images from one location to another.

Another object is :to :provide an improved method of '1 :making fiber optical .devices of the above character having greater efliciency of operation resulting from the prowision of novel means and method for=preventing said devices 'from receiving and/or "transmitting extraneous light while in use.

Other objects and advantages of the:invention wil1 become apparent from the following description when ttakcnin conjunction withythe accompanying drawings in which:

1 is ;a diagrammatic illustration of and method for performing aninitial step :in the forming ofa glevice having the character of the invention;

FIG. 2 ,is a transverse cross-sectional view of alightice FIG. 7 is a side elevational view of amodification of the device of theinvention;

FIG. 18 is.-a;side elevationalviewof a further modified form of .the device of the invention; and

.9 is an enlarged longitudinal cross-sectional view of .a tapered light-conducting fiber and further diagram- ,matically illustrates the manner :in which light rays .are conducted through the same.

.Referring more particularly to the drawings wherein like characters of reference designate like parts through- Qutthe several views thereof, there is diagrammatically illustrated in FIG. :1, a drawing technique by which a ,plurality of light conducting elements or fibers 10 may be-formed .into a conically shaped or otherwise similarly .tapered tightly ;packed ,group or bundle from which :a fiber optical .magnifier :or demagnifier of the character- .of the:invention is formed.

The fibers.10 are shown in the drawings as beingrela- .tivelylarge in cross-sectional area for purposes of better illustrating the novel features of the invention. However, it .should be clear that the size of the fibers 10 would .be selected in accordance with the imageresolw- .i ng powerdesired of the finally formed image transfer .device. which are formed of a multiplicity of closely paclmd .light-conducting elements or fibers, the use :of smaller .fibers in .greaterrnumbersper .unit area will produce the .liigler degrees of resolution .of images directed through such devices.

fibers 10 are each comprised of an inner core .11.of a relatively high index flint glass or the like .and an outer coating or cladding 12 of low index glass having a high .borax content such as a borosilicate glass or the The fibers 10 are initially formed by providing -.a ,rodlike member of the high index glass and placing a coating or envelope of the low index glass around said .rorl whereupon the assembly is drawn down to adesired fiber or filament ,size. .-It is pointed out that although the fibers 10 have been illustrated in the drawings as being circular in cross-sectional shape, they may alternatively be square,.hexagonal, octagonal or of any other desired .crossrsectional configurations. ;Inithe forming of .a tapered bundle of light-conductin .fibers of the above character as shown in FIG. 1, a group of fibers 10 which are initially substantially uniformly dimensioned throughout their length .are assembled together in side-by-side parallel connected relation with each other to form a composite tightly packed bundle of "fibers. This initial assembly of the fibers 10 maybe accomplished in the manner shown and described in an application filed February'14, 1958, in the name of Wilfred P. Bazinet, Jr., and bearing Serial'No. 715,406, or by any other known technique.

One end'of the bundle of fibers isthen clamped Within a'stationary clamping member 13, 1, or otherwise held in fixed relation with a ring-like heating element '1 4 in such a manner as to cause its free end to extend substantially centrally through the heating element 14. By heating the bundle of fibers 10 with the element 14 to a suitable drawing temperature in accordance with the 5 types of glasses used to form the fibers 10, the bundle is conducting fiber of .a type which is .used to form "the A device of the invention;

3 is a side elevational view of the device of the invention which diagrammatically illustrates a further r y in'the processing thereof :F QS- 4 d are g atly enlarged a 9- I -drawn away from the heating element 14 as indicated "by -arrow 15 to form the tapered section illustrated in FIG. 1 between its'end parts. A selected portion of the tapered of the bundle of fibers 10 is then cut or otherwise separated from the drawn bundle of fibers '10 by suitable shearing'means or the like 16 to'form 'animage magnifying member 17, FIG. 3. The opposite ends ofthermeni- .That is, in fiber optical image transfer devices Y 3 her 17 are next semi-finished by further cutting and/or grinding operations to provide flat semi-finished surfaces or faces at the opposite endsthereof which are in substantially parallel relationwith each other and disposed substantially normal to the central longitudinal a'xis'of the member 17 as indicated by the dot-dash lines 18 and 19 in FIG. 3. It is pointed out that the distance or spacing x between the faces 18 and 19 and the locations at which said faces are formed on the member 17 are controlled in accordance with the extent of taper or coning of the member '17 so ts to ultimately provide the magnifying device of the invention with a desired power of magnification. In this respect, the face 19 is first formed at a predetermined location along the member 17 which is such asto provide a light entrance area thereon'of a predetermined cross-sectional size in accordance with the size of object field which is to be covered by the member 17 when used as a magnifier. The face 18 is then formed at a distance a 'x from the face 19 in accordance with thepower of magnification desired of the member 17. The power of magnification will'of course increase proportionately as 'the distance x is increased, since the comparative area sizes of the faces'18 and 19 will determine the power of magnification of the member 17. If theface 18 is constructed to be twice the size of the face 19, a 2X magnifier will obviously be formed. The extent of taper or coning to which the member 17 is initially formed will, as stated previously, be a controlling factor in the resultant thickness x of the member 17 since by increasing the severity of taper or coning of the fibers 10 the distance x may be proportionately decreased without changing the power of magnification of the magnifier.

Following the forming of the semi-finished faces 18 and 19 on the magnifying member 17, the cladding or insulating glass layer 12 which surrounds the cores 11 of the fibers 10 is recessed below the semi-finished ends of the cores 11, as shown in FIG. 4, by etching or otherwise removing a substantial amount of the insulating material 12 from around the cores 11 thereof. The etching of the material 12 is accomplished'by immersing or otherwise applying hydrochloric acid or any other suitable etching material to the semi-finished faces 18 and 19 of the member 17 and allowing the etching material to erode a predetermined amount ofthematerial 12 so as to recess said material well below the level of the cores 11 of the fibers 10.

By initially providing a borosilicate glass coating 12 around the flint glass cores 11 of the fibers 10, as discussed above, hydrochloric acid may be successfully used to etch the material 12 in the manner illustrated in FIG. 4 without causing any appreciable etching or damage to adjacent ends of the flint glass cores 11.

A light reflective coating 20, FIG. 5, of silver,aluminum or any suitable highly reflective material is next applied to the faces 18 and 19 of the member 17 by any conventional technique such as the well known process of coating by evaporation or by simply painting the faces 18 and 19 with the reflective material.

Since the opposite ends of the cores 11 of the fibers 10 must be cleared and optically finished to receive and emit image forming light, the faces =18 and 19 of the member 17 are polished with a conventional flat polishing tool so as to remove the reflective material 20 from the ends of the cores 11 and to simultaneously optically finish their opposite ends.

Due to the fact that the cladding or insulating material 12 has been recessed as described, the reflective coating 20 between the core parts 11 of the fibers will remain untouched as shown in FIG. 6 and thus prevent light from entering the cladding material 12 through the faces 18 or 19 of the magnifying member 17. a In this manner,

images may be accurately transmitted through the memib'er 17 and received at its light exit and with a high degree bfcontrast. J It'is pointed out that tapered light-conducting fibers or 4 bundles formed thereof have certain peculiarities as compared to the usual straight or parallel side light-conducting fibers or devices formed therefrom. That is, the behavior of light being transmitted by the principle of internali'reflection through tapered Sfibers differs considerably from that of conventional straight light-conducting fibers in that light traveling from the small end of a tapered fiber to its larger end tends to become collimated an amount proportionate to the difierence in cross-sectional area of the opposed ends of such a fiber. Referring more particularly to FIG. 9, for example, if the'half cone angle of a ray of entrance light L at the small end (of diameter d of a tapered fiber is represented asfi; and-the half cone angle of said ray oflight at the exit or large end of 'the fiber (of diameter d yis represented as 0,, it holds true that sin o sin 8 d Since this equation is the same for a conventional lens system of corresponding magnification it is evident that a tapered fiber bundle is the full equivalent of such is lens system of zero working distance at each end. That is, in a tapered fiber bundle, the object and image planes are coincident with the respective faces thereof and the device '17 illustrated in FIG. 6 will, of course, function as a magnifier of zero working distance when an object to be viewed thereby is placed against its smallend or face 19 and conversely as a demagnifier when an object to be viewed is placed against its larger end or face 18.

Since light traveling from the small end to the larger end of a tapered fiber or bundle thereof is collimated as just described, it also holds true that light traveling from the larger end to the smaller end is de-collimated or spread out. When, according to the principle of internal reflection in transilluminators of the above character,

portions of said de-collimated light become spread out beyond the critical angle of reflection of the internal side walls of a particular fiber, said portions are cast loose to travel through the cladding of the fiber as stray light. Thus, it is evident that in the magnifying direction, tapered fibers or bundles thereof are very susceptible to picking up external or stray light and in the demagnifying direction, light is continually shed from the fibers. However, in a bundle of tapered fibers 17 such as shown and described herein all of thelight which is shed in the demagnifying direction from the innermost fibers 10 will not beimmediately shed completely from the assembly or bundle of fibers and may be re-caught as stray light to dilute image forming light being transmitted through the cores 11 of the fibers 10.

By applying the light reflecting material 20 to the ends of the cladding 12 as shown in FIGS. 5 and 6, stray light which, would normally enter or exit through said ends is reflected back by the material 20.

In order to prevent external or stray light from entering the sides of the device 17, it is desirable in many instances to provide a coating or shield 21 of opaque or light absorbent material about the outer side surfaceof said device. The shield 21 may be formed of a coating of metal, opaque or light absorbent glass, or any such suitable material. The shield 21 may be formed integrally with the member 17 by'initially placing the bundle of diffusing plate or screen 22 as illustrated in FIG. 7 so as to cause the image forminglight emitted from the fibers -10. :toilluminate saidscreenfandgbe scattered outwardly thereby in all directions to render all parts of the image received by said screen simultaneously visible to the eye of an observer.

In FIG. 8 there is shown further modified means for causing all parts of an image emitted at the light exit end or face 23 of a fiber optical magnifier 24 to be clearly visible at a predetermined eye point 25. The outermost fibers 10 at the light exit face 23 of the magnifier 24 are curved inwardly so as to converge slightly toward each other an amount suflicient to cause substantially all of the image forming light emitted therefrom to be directed gosward the predetermined viewing location or eye point The magnifier 24 is constructed in a manner similar to the magnifier 17 with the exception that the convergent end or face 23 is initially formed by reversing a bundle of fibers 10 such as illustrated in FIG. 1 following the drawing operation described hereinabove and securing the tapered part thereof in the clamp 13 whereupon the bundle is reheated and drawn from its opposite end an amount sufiicient to produce a slight taper or coming at its light exit end 23. The device 24 is then cut from the drawn bundle at predetermined locations above and below its widest part to form its light entrance and exit aces.

It should be understood that since fiber optical devices of the above character have zero working distances, the hght entrance and exit ends or faces thereof may be formed to any desired shape other than the flat surfaces llustrated the drawings. That is, the objective or image receiving end of the device 17 may be shaped to correspond to the shape of the plane of the image which s to be projected through the device 17 whether said image plane is fiat, concavely or convexly curved or otherwise contoured. Moreover, the light exit end of the device 17 may be contoured to any desired shape in accordance with the shape desired of the surface in which the transferred image is to be received.

It is also pointed out that bundles of light-conducting fibers which are not tapered may be treated in the manner discussed hereinabove to prevent light from being transmitted through the coating or cladding of the fibers so as to improve the contrast of images transferred by such bundles.

It is further pointed out that instead of initially providing each core with a low'index coating the said low index material may be assembled with the cores as spacer members between said cores. Such spacer members may be fused in surrounding relation with the cores during the drawing operation and will thereafter function as spacer mtgans to retain said cores in spaced relation with each 0 er.

From the foregoing it can be seen that a simple, economical and highly efiicient method has been provided for accomplishing all of the objects and advantages of the invention. However, it should be apparent that many changes in the details of construction or steps in the method may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims and the invention is not limited to the exact matters shown and described herein as only the preferred matters have been given by way of illustration.

Having described our invention, we claim:

1. The method of making a fiber optical device of the character described comprising providing a bundle formed of a multiplicity of cores of light-conducting material having a relatively high index of refraction, said cores being spaced and held in bundled relation by a lightconducting material having a relatively low index of refraction, etching at least one of the bundle ends of said spacing material without appreciably affecting said cores, applying a light shielding material to the etched spacing material at said bundled end, and thereafter polishing the ends of said cores for optically finishing said core ends while permitting said light-shielding material to re- 16 main on the etched spacing material-at said etched bundle end, thereby preventing light from entering the bundle through said spacing material at said end. i

2. The method of making a fiber optical device ofthe character described comprising providing a bundle formed of a multiplicity of tightly-packed and connected members each of which comprises a core of light-conducting material having a relatively high index of refraction and an outer coating of light-conducting material having a relatively low index of refraction, etching the ends of said coatings without appreciably affecting said cores, applying a layer of light-reflecting material to the bundle ends, and abrading the bundle ends for removing said light-reflecting material from the ends of said cores and for optically finishing said core ends while permitting said light-reflecting material to remain on the etched ends of said coatings.

3. The method of making a fiber optical device of the character described comprising providing a bundle formed of a plurality of tightly-packed and connected members each of which comprises a core of light-conducting material having a relatively high index of refraction and an outer coating of light-conducting material having a relatively low index of refraction, etching the ends of said bundle without appreciably afiecting said cores, applying a layer of light-reflecting material to the sides and ends of the bundle, and abrading the bundle ends for removing said light-reflecting material from the ends of said cores and for optically finishing said core ends while permitting said light-reflecting material to remain on the etched ends of said coatings.

4. The method of making a magnifying and demagnifying fiber optical device of the character described com prising providing a plurality of light-conducting fibers each embodying a core formed of a light-conducting material having a relatively high index of refraction and an outer coating of light-conducting material having a relatively low index of refraction, assembling said fibers in sideby-side relation for forming a bundle, heat-softening the fibers, drawing the bundle, from one end in the direction of its longitudinal axis to cause said fibers to become substantially uniformly tapered in connected relation with each other throughout at least a predetermined section of said bundle, cutting the bundle transversely of its longitudinal axis for removing said predetermined section, etching the ends of said coatings without appreciably affecting said cores, applying a layer of light-reflecting material to the ends of the section, and abrading the ends of the section for removing said light-reflecting material from the ends of said cores and for optically finishing said core ends while permitting said light-reflecting material to remain on the etched ends of said coat- 5. The method of making a fiber optical device as set forth in claim 4 comprising the further steps of providing a light-difiusing member, and mounting said member at one end of said bundle for forming an image viewing screen at said bundle end.

6. The method of making a magnifying fiber optical device of the character described in which an image emitted from the device is clearly visible at a predetermined eye point, said method comprising as steps: providing a multiplicity of light-conducting fibers each of which embodies a core formed of a light-conducting material having a relatively high index of refraction and an outer coating of light-conducting material having a relatively low index of refraction; assembling said fibers in side-by-side relation for forming a bundle; heat-softening the fibers for fusing said coatings; drawing the bundle from one end in the direction of its longitudinal axis for forming a section of predetermined length extending from said bundle end; cutting the bundle transversely of said axis adjacent said end for providing a light entrance face; drawing the bundle from the other end in the direction of said axis for forming a section of predetermined length in which the axes of said cores are adapted to converge at said eye point; cutting the bundle transversely of said adjacent said other end for providing a light emitting trace; etching the ends of said coatingsat said faces without appreciably affecting said cores; applying a layer of light-reflecting-ma-terial to said faces; and abrading said faces for removing the light-reflecting material from said core ends and for optically finishing said core ends while permitting said light-reflecting material to remain on the ends of said etched coatings. I i

7. The method of making a fiber optical device of the character described comprising providing a bundle formed of tightly-packed and connected members each embodying a core of light-conducting material having a relatively high index of refraction and an outer coating of borosilicate glass having a relatively low index of refraction, etching the ends of said bundle with hydrochloric acid to remove said coatings from the ends of said cores without appreciably affecting said cores, applying a layer of light-reflecting material to the ends of said bundle, and abrading the bundle ends for removing said light-reflecting material from said core ends and for optically finishing said core ends While permitting said lightreflecting" material to remain on the etched ends of said coatings.

References Cited in the file of this patent UNITED STATES PATENTS 2,328,302 Simison Aug. 31, 1943 2,510,106 Henroteau June 6, 1950 2,825,260 OBrien Mar. 4, 1958 

